Did you read the books? (This isn't meant to sound condescending, it sounds that way I know, but that isn't the intent) The reason I ask is because Thoros is very jovial and goofy early for certain. The real Thoros shows up around the time that the show is reaching. In the time of peace during Robert's reign he got fat and fairly lazy same as most.

Once he gets back into his element of being a warrior of R'hllor he becomes much more serious. Don't forget that Thoros of Myr led the charge through the breach during the Greyjoy's rebellion. He is a terrible foe indeed, charging forth with a sword alight with fire.

The Thoros in my mind was never that dark until after the events of ASOS and "the change" in the brotherhood. Before that I imagined him to be like the cast video. Kind of goofy looking man trying to do what's right. Not a complete fool but not taking the situation seriously either. First in battle, and first in drinking